Aromatic polyimide resins are useful engineered plastics having the rigidity and resonance stabilization of their molecular chains as well as high thermal stability, high strength, and high solvent resistance because of their strong chemical bonds. The aromatic polyimide resins, however, are generally difficult to mold by heat fusion due to their high thermal stability. Also, their molded products are generally difficult to obtain via a polyimide solution due to their high solvent resistance. Thus, methods adopted for producing the molded products of the aromatic polyimide resins involve reacting a tetracarboxylic acid component with a diamine component to prepare a polyamic acid, and molding the polyamic acid at this stage, followed by the imidization of the polyamic acid by various methods. One of the most known methods is a method which involves reacting an aromatic tetracarboxylic acid component with a diamine containing an aromatic ring in a high-boiling point solvent such as N-methylpyrrolidone or dimethylacetamide to prepare a polyamic acid, molding the polyamic acid into a film form on a support, and then thermally imidizing this polyamic acid under high temperature conditions to obtain a polyimide film (see PTL1).
This method, however, requires a plurality of labors, i.e., requires (1) uniform volatilization of the solvent for molding the polyamic acid into a film form, and (2) a high temperature exceeding 300° C. in the subsequent imidization.
However, some aromatic polyimide resins can be molded by recovery in a powder state and subsequent exposure to high temperature and high pressure for a long time (see PTL2).
On the other hand, among semi-aromatic polyimide resins or fully aliphatic polyimide resins, some polyimide resins have been reported to have solvent solubility or thermoplasticity. The solvent-soluble polyimide resins can be molded into films from polyimide varnish, and the thermoplastic polyimide resins can be molded into various forms by heat fusion. These polyimide resins having molding processability can be molded even if the resins are obtained in forms that are not final product forms, i.e., in a powder or clump form. Thus, such semi-aromatic polyimide resins or fully aliphatic polyimide resins are superior to the aromatic polyimide resins from the viewpoint of molding processability.
Since there exist a large number of molding process techniques for polyimide resin powders, the polyimide resin powders are highly versatile. Particularly, if polyimide resins can be recovered as homogeneous powders, these powders are highly useful because uneven processing is less likely to occur during the molding process. The polyimide resins in a powder form also have advantages such as high storage stability and easy delivery.
PTL3 has reported a method for synthesizing a polyimide resin at normal pressure and recovering the polyimide resin as a powder. PTL3 has reported that an aromatic tetracarboxylic acid and a linear aliphatic diamine are heated at a temperature on the order of 160° C. in N-methyl-2-pyrrolidone and toluene to form a polyimide resin in a powder form. This reaction is also called imidization under heating to reflux, which is also characterized in that a polyimide resin is deposited as a powder.
The synthesis method by the imidization under heating to reflux, however, can be used actually only in rare cases as a method for synthesizing a polyimide resin using an aliphatic diamine. In general, N-methyl-2-pyrrolidone, dimethylacetamide, or the like is used as a polymerization solvent for polyimide resins. The synthesis of semi-aromatic polyimide resins in these solvents presents many problems, for example: (1) a strong salt of the tetracarboxylic acid and the aliphatic diamine and/or a strong salt of the polyamic acid and the aliphatic diamine is formed so that a large amount of deposits are generated at the initial stage of the reaction, thereby causing poor stirring or inhomogeneous reaction; (2) although the polyimide resin is recovered as a deposited solid, this solid forms large clumps due to unstable particle sizes or has poor filterability due to its too small particles; and (3) since the reaction time is several hours or longer, productivity cannot be secured. Hence, it must be said that this synthesis approach is poorly practical. In fact, the polyimide resin prepared using an aliphatic diamine is distributed in a very small amount on the market because of its difficult synthesis and low productivity.